You could write yourself two macros:

HalfPageDown.bsh
// Scroll the current text area down half a page, without altering position
// of caret

int firstLine=textArea.getFirstPhysicalLine();
int lines=textArea.getVisibleLines();
int newFirstLine=firstLine-(lines/2);

textArea.setFirstPhysicalLine(newFirstLine);

HalfPageUp.bsh

// Scroll the current text area up half a page, without altering position
// of caret
int firstLine=textArea.getFirstPhysicalLine();
int lines=textArea.getVisibleLines();
int newFirstLine=firstLine-(lines/2);
if (newFirstLine<0)
{
newFirstLine=0;
}
textArea.setFirstPhysicalLine(newFirstLine);
textArea.setFirstPhysicalLine(firstLine+(lines/2));



You can then use Utilities-Global Options-Shortcuts - (Edit Shortcuts: Macros) to assign these two macros to C+PAGE_DOWN and C+PAGE_UP

Then pressing Control and Page Down will scroll the current text area half a page, and likewise Control and Page Up will scroll back up half a page.

This at least appears to work on 4.2pre9

hth